diff --git a/packages/http-client-csharp/emitter/src/lib/type-converter.ts b/packages/http-client-csharp/emitter/src/lib/type-converter.ts
index ded35eb85e8..23d9346badb 100644
--- a/packages/http-client-csharp/emitter/src/lib/type-converter.ts
+++ b/packages/http-client-csharp/emitter/src/lib/type-converter.ts
@@ -118,12 +118,18 @@ export function fromSdkType<T extends SdkType>(
         typeof sdkProperty.serializedName === "string" &&
         sdkProperty.serializedName.toLocaleLowerCase() === "content-type";
 
+      // Check if the original property type was an enum (e.g., when @alternateType collapses
+      // a single-member enum to a constant) - preserve the enum type in this case
+      const isOriginalTypeEnum =
+        sdkProperty !== undefined && sdkProperty.__raw?.type?.kind === "Enum";
+
       if (
         sdkProperty &&
         !isContentTypeHeader &&
-        (sdkProperty.optional || sdkProperty?.type.kind === "nullable") &&
-        sdkProperty?.type.kind !== "boolean" &&
-        sdkType.valueType.kind !== "boolean"
+        (((sdkProperty.optional || sdkProperty?.type.kind === "nullable") &&
+          sdkProperty?.type.kind !== "boolean" &&
+          sdkType.valueType.kind !== "boolean") ||
+          isOriginalTypeEnum)
       ) {
         // turn the constant into an extensible enum
         retVar = diagnostics.pipe(createEnumType(sdkContext, sdkType, namespace!));
diff --git a/packages/http-client-csharp/emitter/test/Unit/constant-type.test.ts b/packages/http-client-csharp/emitter/test/Unit/constant-type.test.ts
index 3730f08dbdd..46f38ad8c1b 100644
--- a/packages/http-client-csharp/emitter/test/Unit/constant-type.test.ts
+++ b/packages/http-client-csharp/emitter/test/Unit/constant-type.test.ts
@@ -149,4 +149,34 @@ describe("Constant enum conversion", () => {
     strictEqual(enumType.access, undefined);
     strictEqual(enumType.usage, UsageFlags.Input | UsageFlags.Json);
   });
+
+  it("single-member enum with @alternateType should preserve enum type for required property", async () => {
+    const program = await typeSpecCompile(
+      `
+      enum Colors {
+        green,
+      }
+
+      model TestModel {
+        @alternateType("green")
+        prop: Colors;
+      }
+
+      op test(@body input: TestModel): void;
+    `,
+      runner,
+      { IsTCGCNeeded: true },
+    );
+    const context = createEmitterContext(program);
+    const sdkContext = await createCSharpSdkContext(context);
+    const [root] = createModel(sdkContext);
+    const testModel = root.models.find((m) => m.name === "TestModel");
+    ok(testModel);
+    const propertyType = testModel.properties[0].type;
+    strictEqual(propertyType.kind, "enum");
+    const enumType = propertyType as InputEnumType;
+    strictEqual(enumType.isFixed, false);
+    strictEqual(enumType.values.length, 1);
+    strictEqual(enumType.values[0].value, "green");
+  });
 });